Synthesis of CdSe and CNT-CdSe composite Synthesis of CdSe For the synthesis of CdSe compound, the sodium seleno sulfite Na2SeSO3 solution and CdNH342+solution was prepared at first.. Fo
Trang 1N A N O E X P R E S S Open Access
Synthesis and highly visible-induced
photocatalytic activity of CNT-CdSe composite
for methylene blue solution
Abstract
Carbon nanotube-cadmium selenide (CNT-CdSe) composite was synthesized by a facile hydrothermal method derived from multi-walled carbon nanotubes as a stating material The as-prepared products were characterized by X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, transmission electron
microscopy (TEM), and UV-vis diffuse reflectance spectrophotometer The as-synthesized CNT-CdSe composite efficiently catalyzed the photodegradation of methylene blue in aqueous solutions under visible-light irradiation, exhibiting higher photocatalytic activity
Introduction
Environmental problems such as toxic organic
pollu-tants provide the impetus for fundamental and applied
research into environmental areas Semiconductor
photocatalysts have attracted considerable attention for
a long time in the fields of photochemistry [1-5] because
of their usefulness with regard to solving environmental
problems Over the last few years, considerable efforts
have been made in the controlled synthesis of various
nanoscaled materials to improve their properties for
photocatalysis Cadmium selenide (CdSe) is an n-type
semiconductor Its bandgap energy was reported to be
in the range from 1.65 to 1.8 eV [6-9] CdSe was found
to be suitable for various optoelectronic applications
such as light-emitting diodes, laser diodes [10-13],
cata-lysis [14], solar cells [15], and biological labeling [16]
More recently, many groups have synthesized CdSe
nanomaterials with high photocatalytic activity in the
degradation of organic pollutants under UV light
irra-diation, such as CdSe-Pt nanorods and nanonets [16],
hybrid CdSe-Au nanodumbbells [17],
CdSe/ZnS-photo-sensitized nano-TiO2 film [18] Therefore, as an
impor-tant semiconductor, CdSe is an effective catalyst for
photocatalytic degradation of organic pollutants
How-ever, a few recent papers have discussed the preparation
and properties of CdSe combining with carbon nano-tubes (CNTs) composite Since the discovery of the CNTs [19,20], they have attracted much attention because their unique mechanical, optical, and electrical properties that may impact many fields of science and technology [21-24] However, the functionalization of CNTs requires chemical modification of their surface, in order to form the functional groups on the surface
In this paper, the multi-walled carbon nanotubes (MWCNTs) were used as start material and functionalized
by m-chlorperbenzoic acid (MCPBA) Then the CNT-CdSe composite were prepared directly via a conventional hydrothermal method The intrinsic characteristics of resulting composite were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX), transmission electron microscopy (TEM) analysis and UV-vis diffuse reflectance spectro-photometer The photocatalytic activity of the as-synthe-sized samples was evaluated by degrading methylene blue (MB) under irradiation of visible light
Experimental
Materials Crystalline MWCNTs powder (diameter, 5~20 nm; length, ~10μm) of 95.9 wt.% purity from Carbon Nano-material Technology Co., Ltd., Pohang-si, Gyungbuk-do, Korea was used as a starting material For the oxidiza-tion of MWCNTs, MCPBA was chosen as the oxidizing agent which purchased from Acros Organics, New
* Correspondence: wc_oh@hanseo.ac.kr
Department of Advanced Materials Science & Engineering, Hanseo
University, Seosan-si, Chungnam-do, 356-706, Korea
© 2011 Chen and Oh; licensee Springer This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in
Trang 2Jersey, USA Benzene (99.5%) was used as the organic
solvent which purchased from Samchun Pure Chemical
Co., Ltd, Seoul, Korea Cadmium acetate dihydrate (Cd
(CH3COO)2, 98%), selenium metal powder, and
ammo-nium hydroxide (NH4OH, 28%) were purchased from
Dae Jung Chemicals & Metal Co., Ltd, Siheung-si,
Gyonggi-do, Korea Anhydrous purified sodium sulfite
(Na2SO3, 95%) was purchased from Duksan
Pharmaceu-tical Co., Ltd, Ansan-si, Gyeonggi-do, Korea The MB
(C16H18N3S·Cl, 99.99+%) was used as model pollutant
which purchased from Duksan Pure Chemical Co., Ltd,
Ansan-si, Gyeonggi-do, Korea All chemicals used
with-out further purification and all experiments were carried
out using distilled water
Synthesis of CdSe and CNT-CdSe composite
Synthesis of CdSe
For the synthesis of CdSe compound, the sodium seleno
sulfite (Na2SeSO3) solution and Cd(NH3)42+solution was
prepared at first Na2SO3(5 g) and selenium metal
pow-der (0.5 g) were dissolved in 30-mL distilled water and
refluxed for 1 h to form Na2SeSO3solution Meanwhile,
Cd(CH3COO)2(0.5 g) was dissolved in 2-mL distilled
water NH4OH (6 mL) was added to it and the mixture
was stirred till it dissolved completely to form Cd(NH3)4
+
solution Finally, Cd(NH3)42+and Na2SeSO3solutions
were mixed together and the mixture was stirred and
refluxed for at least 5 h After the temperature of the
mixture was brought down to room temperature, the
mixture was filtered through Whatman filter paper The
solid obtained was collected and washed with distilled
water for five times After being dried in vacuum at 353
K for 8 h, the CdSe compound was obtained
Synthesis of CNT-CdSe composite For preparation of the CNT-CdSe composite, the MWCNTs had to functionalize by MCPBA at first MCPBA (1 g) was melted in 60 mL benzene, and then 0.5 g MWCNTs was put into the oxidizing agent The mixture was stirred with a magnet for 6 h at 343 K Then the MWCNTs was dried at 373 K and spared The functionalized MWCNTs with Cd(NH3)42+ and
Na2SeSO3 solutions which were prepared above were mixed together and the mixture was stirred and refluxed for at least 5 h After the temperature of the mixture was brought down to room temperature, the mixture was filtered through Whatman filter paper The solid obtained was collected and washed with distilled water for five times After being dried in vacuum at 353 K for
8 h, the CNT-CdSe composite with chemical band was obtained Figure 1 shows the schematic presentation of the functionalization of MWCNTs and the coupling of CdSe nanoparticles with MWCNTs
Characterization XRD (Shimadz XD-D1, Uki, Kumamoto, Japan) result was used to identify the crystallinity with monochro-matic high-intensity CuKa radiation (l = 1.5406 Å) SEM (JSM-5600, JEOL Ltd., Tokyo, Japan) was used to observe the surface state and structure of prepared com-posite using an electron microscope Transmission elec-tron microscopy (TEM, Jeol, JEM- 2010, Japan) was used to determine the state and particle size of prepared composite TEM at an acceleration voltage of 200 kV was used to investigate the number and the stacking state of graphene layers on various samples TEM speci-mens were prepared by placing a few drops of sample
Figure 1 Schematic presentation of the functionalization of MWCNTs and the coupling of CdSe nanoparticles with MWCNTs.
Trang 3solution on a carbon grid The element mapping over
the desired region of prepared composite was detected
by an EDX analysis attached to SEM UV-vis diffuse
reflectance spectra were obtained using an UV-vis
spec-trophotometer (Neosys-2000, Scinco Co Ltd., Seoul,
Korea) by using BaSO4 as a reference at room
tempera-ture and were converted from reflection to absorbance
by the Kubelka-Munk method
Photocatalytic activity measurements
The photocatalytic activity under visible lamp (KLD-08L,
220 V, 50-60 Hz, 8 W, pure white, l > 420 nm, Fawoo
Tech Co., Ltd., Tokyo, Japan) irradiation of the
CNT-CdSe composite was evaluated by using MB as the
model substrate In an ordinary photocatalytic test
per-formed at room temperature, 0.05 g CNT-CdSe
compo-site was added to 50 mL of 1.0 × 10-5-mol/L MB
solution, which was hereafter considered as the initial
concentration (c0) Before turning on the visible lamp,
the solution mixed with composite was kept in the dark
for at least 2 h, allowing the adsorption/desorption
equi-librium to be reached Then, the solution was irradiated
with visible lamp The first sample was taken out at the
end of the dark adsorption period (just before the light
was turned on), in order to determine the MB
concen-tration in solution after dark adsorption, which was
hereafter considered as the initial concentration (cads)
Samples were then withdrawn regularly from the reactor
by an order of 30, 60, 90, 120, 180, and 240 min, and
immediately centrifuged to separate any suspended
solid The clean transparent solution was analyzed by
using a UV-vis spectrophotometer (Optizen POP,
Mecasys Co., Ltd, Seoul, South Korea) at wavelength of
665 nm [25-27]
Results and discussion
Characterization
Figure 2 shows X-ray patterns of the pristine MWNTs,
CdSe, and CNT-CdSe composite From Figure 2, it can
be seen that the diffractogram of pure MWCNTs exhibit
the typical peaks at 25.9° and 42.7°, corresponding to the
graphite (002) and (100) reflections (Joint Committee for
Powder Diffraction Studies (JCPDS) No 01-0646) [28],
respectively For CdSe compound, the XRD diffraction
peaks around 2θ of 25.4°, 42°, and 50°, which can be
indexed to the characteristic peaks (111), (220), and
(311) plane reflections of cubic crystal structure CdSe
with lattice constants of 6.05 Å according to the
stan-dard powder diffraction data (JCPDS No 65-2891 for
CdSe, cubic) [29,30] However, for CNT-CdSe
compo-site, only the typical peaks arose from CdSe were
detected As we known, CdSe has three crystalline
forms wurtzite (hexagonal), sphalerite (cubic), and
rock-salt (cubic) The sphalerite CdSe structure is unstable
and converts to the wurtzite form upon moderate heat-ing The transition starts at about 130°C, and at 700°C it completes within a day The rock-salt structure is only observed under high pressure However, in our study, the highest temperature was 70°C to approximately 80°C
at hydrothermal experiment So the obtained CdSe com-pound and CNT-CdSe composite exited cubic CdSe structure Therefore, the micromorphology of CNT-CdSe is different from that of the mixture of MWNTs and CdSe No peaks for impurities are detected, indicat-ing that the hydrothermal method used in this study is responsible for the formation of the CNT-CdSe composite
Figure 3 shows the SEM microphotographs of CdSe and CNT-CdSe composite From the Figure 3a, very uniform spherical-shaped CdSe particles with agglomer-ate together can be observed For CNT-CdSe composite,
as shown in Figure 3b, spherical-shaped agglomerated CdSe particles are mixed with MWCNTs More detailed information of the surface state can be confirmed by the transmission electron microscopy (TEM) Figure 4 shows the TEM image of CNT-CdSe composite It can
be observed that the surface of MWCNTs have been coated with CdSe layers uniformly with particle size of about 10 nm
To get information about change in elements and ele-ment weight percent, the prepared CdSe and CNT-CdSe composite were examined by EDX Figure 5 shows the EDX microanalysis and element weight percent of CdSe and CNT-CdSe composite From Figure 5a, b, main ele-ments such as Cd and Se are existed in CdSe composite Apart from these two kinds of main elements, the main element C is also existed in CNT-CdSe composite, as shown in Figure 5c, d
0 100 200 300 400 500 600 700 800
(002)
(100)
(311) (220)
2-Theta ( o )
MWCNTs CdSe compound CNT-CdSe composite ((111111))
Figure 2 XRD patterns of MWCNTs, CdSe, and CNT-CdSe composite.
Trang 4Figure 6 shows the UV-vis diffuse reflectance spectra
of CdSe and CNT-CdSe composite The reflectance
characteristics of the CdSe composite were quite similar
to that of the CNT-CdSe composite except the CdSe
composite has an absorption edge at 830 nm We can
use the following formula to calculate the band gap energy of CdSe
where a, v, Eg, and A are the absorption coefficient, light frequency, band gap, and a constant, respectively Therefore, the band gap energy (Eg) of CdSe compound can be estimated from a plot of (ahv)1/2
versus photo energy (hv), as shown in figure inset in Figure 6 The band gap energy of CdSe is 1.74 eV, which is fairly close
to literature value of 1.65 to 1.8 eV (CdSe) [6-9] Moreover, the two composite both exhibit strong absorption in the UV light region with wavelength less than 400 nm and visible-light region with wavelength at 400-800 nm, assigned to the band adsorption of CdSe And the absorption of CNT-CdSe composite is higher than that of CdSe compound in both of UV light and visible-light region, as the MWCNTs act as good electron acceptors can accept the electrons from light irradiation [31,32], indicating the CNT-CdSe composite would exhi-bit more excellent photoactivity than CdSe compound Degradation of MB solution
The photocatalytic activities of the CNT-CdSe compo-site were evaluated by the photodegradation of MB aqu-eous solution under visible-light irradiation The decreasing concentration of MB in the photocatalytic reaction was used to evaluate the activity of the compo-site The characteristic absorption peak of MB solution
at 665 nm was chosen as the monitored parameter to detect the concentration of MB solution
Figure 7 represents the degradation of MB over CNT-CdSe composite with amount of 0.05 g as a function of the original MB concentration under visible-light irradia-tion For different concentrations of the original MB aqu-eous solution, the level of photodegradation is quite different after 240 min illumination After illumination for 240 min, the adsorption efficiency of the 1 × 10-5 mol/L and 5 × 10-5mol/L was 91% and 54%, respectively However, for the 1 × 10-4mol/L and MB concentration, only about 10% was degraded after 240 min Therefore, it seems that the photodegradation efficiency of the MB photocatalyzed by the CNT-CdSe composite decreased
as the original MB concentration increased The main reason is that the initial dye concentration may affect strongly the rate of the photocatalytic process
Figure 8 shows the effect of the amount of the CNT-CdSe composite on the photocatalytic performance under visible-light irradiation The concentration of MB solution is 1×10-5 mol/L From the Figure 8, it is obvious that 0.05 g of the CNT-CdSe composite gave the best results of photodegradation of MB solution And the photodegradation efficiency of the MB
(a)
(b)
Figure 3 SEM microphotographs of CdSe (a) and CNT-CdSe (b)
composite.
Figure 4 TEM image of CNT-CdSe composite.
Trang 5photocatalyzed by the CNT-CdSe composite decreased
as the amount of the CNT-CdSe composite increased
Figure 9 represents the degradation of MB over CdSe
compound and CNT-CdSe composite under visible-light
irradiation, the MB concentration is 1×10-5mol/L; the
amount of CdSe compound and CNT-CdSe composite
is 0.05 g We can clearly see that the concentration of the MB solution gradually diminish with increasing irra-diation time for all of samples After irrairra-diation for 240 min, the CdSe compound has almost no photocatalytic
(a) Quantitative results
0 10 20 30 40 50
O Na S Cu Zn Se Cd
(b)
(c)
Quantitative results
0 10 20 30 40
C Na Fe Cu Zn Se Cd
(d)
Figure 5 EDX microanalysis and element weight percentageof CdSe ((a) and (b)) and CNT-CdSe ((c) and (d)).
Trang 6activity toward the photodegradation of MB solution.
The presumed reason is that a mass of visible light may
be absorbed by the MB molecules in aqueous solution
rather than the CdSe particles for high MB
concentra-tion, which can reduce the efficiency of the catalytic
reaction However, for CNT-CdSe composite, a much
excellent photocatalytic activity toward the
photodegra-dation of MB solution can be observed and the MB
con-centration is removed 55% after irradiation under visible
light for 240 min
According to the above experimental data, the
CNT-CdSe composite has an excellent photocatalytic activity
toward the photodegradation of MB solution under
visi-ble-light irradiation Figure 10 shows the MWCNTs
act-ing an electron acceptor for improving the
photocatalytic activity of CdSe compound Under
irradiation by visible lamp, the MWCNTs acting as good electron acceptors can accept the electrons by light irradiation [31,32] Meanwhile, the CdSe can be also excited to produce the electrons and holes in the conduction band (CB) and valence band of CdSe Then the electrons accepted by MWCNTs from light can transfer into the CB of CdSe, thereby increasing the number of electrons as well as the rate of electron-induced redox reactions The generated electrons (e-) probably react with dissolved oxygen molecules and pro-duce oxygen peroxide radical O2 •-, the positive charged hole (h+) may react with the OH-derived from H2O to form hydroxyl radical OH• The MB molecule then can
be photocatalytically degraded by oxygen peroxide radi-cal O2 •-and hydroxyl radical OH• to CO2, H2O, and other mineralization [31-34]
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
2.00 1.75 1.50 1.25
0
10
20
30
40
50
60
70
80
eV
Eg (eV)
CdSe compound
Wavelength (nm)
CdSe CNT-CdSe
Figure 6 UV-vis diffuse reflectance spectra of CdSe and
CNT-CdSe composite.
0.0
0.2
0.4
0.6
0.8
1.0
1u10-5 mol/L
5u10-5 mol/L
1u10-4 mol/L
c 0
Reaction time (min)
Figure 7 Dependence of the MB concentration of the
CNT-CdSe composite (0.05 g) under visible light irradiation.
0.0 0.2 0.4 0.6 0.8 1.0
c 0
Reaction time (min)
0.05 g 0.03 g 0.01 g
Figure 8 Degradation of MB under visible light irradiation for the CNT-CdSe composite with different amount.
0.0 0.2 0.4 0.6 0.8 1.0
c 0
Irradiation time under visible light (min)
CdSe compound CNT-CdSe composite
Figure 9 Degradation of MB under visible light irradiation for the CdSe compound and CNT-CdSe composite.
Trang 7In this study, CNT-CdSe composite was successfully
synthesized by a simple hydrothermal method From the
XRD patterns, the cubic crystal structure of CdSe can
be observed TEM image shows that the surface of
MWCNTs has been coated with CdSe layers uniformly
with particle size of about 10 nm The EDX results
reveal the presence of C, Cd, and Se with high content
in prepared composite The diffuse reflectance spectra
suggest the CNT-CdSe composite shows strong
photo-absorption at UV light and visible-light range The
photocatalytic activity of the CNT-CdSe composite is
investigated by degradation of MB in aqueous solution
under visible-light irradiation The results reveal that
CNT-CdSe composite exhibit excellent photocatalytic
activity for degradation of MB solution under
visible-light irradiation
Authors ’ contributions
WCO conceived of the study, and participated in its design and
coordination MLC carried out the experiment, processed the data, wrote
and submitted the manuscript All authors read and approved the final
manuscript.
Competing interests
The authors declare that they have no competing interests.
Received: 24 April 2011 Accepted: 26 May 2011 Published: 26 May 2011
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doi:10.1186/1556-276X-6-398
Cite this article as: Chen and Oh: Synthesis and highly visible-induced
photocatalytic activity of CNT-CdSe composite for methylene blue
solution Nanoscale Research Letters 2011 6:398.
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